The present invention relates to a laser driving circuit utilizing a laser diode, and more specifically, to a laser driving circuit wherein the intensity of the laser beam emitted by the laser diode is regulated.
Conventionally, a laser diode, or a semiconductor laser is utilized as a laser emitting means in a laser beam printer and the like. In such a printer, for example, the laser diode is controlled such that the intensity of the laser beam is regulated to become a predetermined intensity.
In order to control the intensity of the laser beam as above, the laser driving circuit generally has a so-called APC (Automatic Power Control) function.
The laser drive circuit employed, for example, the laser beam printer is driven as follows:
At first, power (voltage) is supplied to the laser drive circuit. At this stage, the laser diode does not emit laser beam. After the power (voltage) has supplied in steady state, a print enable signal is transmitted from a host computer, or the like, to actuate the laser diode to emit the laser beam.
In order to avoid a so-called rush current, in the conventional laser driving circuit, the power should be supplied to the circuit, then followed by the enable signal. Upon receipt of the enable signal, the circuit controls the current flow through the laser diode to emit laser beam. If the rush current flows in the laser diode, the laser diode may deteriorate, or worse, the laser diode may be broken. Thus, the order of supplying power to the circuit, and inputting the print enable signal should be maintained. If this order is not kept, the circuit may fall to control the current flow in the laser diode, and the rush current may result.
The laser beam printer, for example, is usually connected with an external device, such as a hose computer, and is used as an auxiliary device for printing data transmitted from the host computer. Under a situation that power has been supplied to the printer, i.e., to the laser driving circuit, and the print enable signal has also been issued, if the power supply is temporarily turned OFF and then it is turned ON again, the circuit may fail to subside the abrupt current flow in the laser diode since the print enable signal has been kept from being issued.